A conventional multipoint scale will be described using the example of a baby scale shown in FIGS. 4 to 6. A baby scale 1 shown in FIG. 4 has a length of up to 660 mm (about 26 inches) for weighing a baby by laying the baby on a placement table 2. In the placement table 2, the sides are downwardly inclined and an intermediate portion is gently U-shaped. In the center of the inclined surface of one side, there are provided an integrally arranged display 3 such as a liquid crystal display, a power-on and tare switch 4, and a power-off switch 5. Baby scale 1 is a four-point bathroom scale having weight sensor units, one for each of the vicinities of four corners of the placement table 2.
FIG. 5 shows a section along a line connecting the centers of two weight sensor units S1 and S2 disposed at two corners in the longitudinal direction when baby scale 1 is placed on a setting surface G such as a floor. Two other weight sensor units S3 and S4 (not shown) are arranged in the same way at two corners on the other side of scale 1.
The weight sensor unit S1 employs a known weight sensor 6 comprising a strain-sensitive part 6a having a strain gage as disclosed, for example, in U.S. Pat. No. 4,993,506. A power point of the weight sensor 6 is fixed to a casing 9 supporting the placement table 2 with a substantially disk-shaped load transmission plate 7 therebetween. A fulcrum is fixed to a fixed leg 10 with a substantially disk-shaped load support plate 8 therebetween. The weight sensor 6 is covered with a protection cover 11.
During weight measurement with the baby scale 1, as shown in FIG. 6, the load transmission plate 7 is inclined in accordance with deflections of the placement table 2 and the casing 9 due to a load, so that the load is slantingly applied to the weight sensor 6. At this time, since load support plate 8 supporting fulcrums of the weight sensor 6 is fixed to the surface G via fixed leg 10, the fulcrums of the weight sensor 6 are not inclined. Accordingly, because of the inclination of the load applied to the power point of the weight sensor 6, torsion is produced in the strain-sensitive part 6a of the weight sensor 6, generating a moment due to the torsion. The strain gage of the strain-sensitive part 6a is affected by the moment due to the torsion, disadvantageously reducing the accuracy of the weight measurement. Moreover, the linearity of the weight measurement tends to be significantly reduced because the degree of incline changes with the load, varying its influence.
To cancel the torsion of the strain-sensitive part 6a mentioned above, the prior art has provided a scale wherein the fulcrums of the weight sensor 6 are movable, so the entire weight sensor 6 is inclined in accordance with the deflecting direction of the placement table 2 and the load applied to the placement table 2 is always perpendicular to the strain-sensitive part 6a of the weight sensor 6. A known movable mechanism including a hardball and a spherical recess arranged between the load support plate 8 and the fixed leg 10 is used so that the load support plate 8 is freely inclined by the hardball rolling on the spherical recess in accordance with the inclination. However, as the area and/or flexibility of the placement table 2 increases, the strain produced in the placement table 2 increases and its inclination is also increased, so that displacement may be generated in the movable mechanism, reducing accuracy in the measurement.
Another conventional scale has a placement table 2 that is substantially a rigid body in material or structure, so that the deflection is suppressed to reduce the inclination of the load applied to the strain-sensitive part 6a. However, the scale itself is heavy and costly. Moreover, since the weight sensor units S1, S2, S3, and S4 are independently attached to the casing 9, if the placement table 2 is laterally pressed, for example, positional displacements of the fixed leg 10 and the load support plate 8 relative to the setting surface G may be independently generated in the weight sensor units S1, S2, S3, and S4, depending on the initial position of the baby scale 1. The displacement of the fulcrum of each weight sensor 6 from their initial state due to these positional displacements generates a torsional moment or a bending moment in each strain-sensitive part 6a. Since the above-mentioned moment is different in each of the weight sensor units S1, S2, S3, and S4, the balance between the four sides and the linearity of a Wheatstone bridge composed of the four weight sensors 6 may deteriorate, adversely affecting measurement accuracy.